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Dual personality of glass explained at last

By Colin Barras

The five-fold symmetry of icosahedral structures means they cannot form the regular 3D crystal lattices characteristic of solids

(Image: Wikimedia Commons)

Although glass feels like a solid, its molecules cannot quite settle into a regular 3D lattice, instead taking on the disordered arrangement of a liquid. Quite why glass behaves like this has been unclear.

Now, however, researchers now say they have found out how it gets its unusual properties. The study could pave the way to developing new materials that combine the best properties of metals and glasses.

An international team has used a gel packed with plastic particles just 2 micrometres across to simulate what happens when a glass cools.

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The particles behave in the same way as the molecules in a glass, and are large enough to be easily visible under a microscope, says research team member Patrick Royall at the University of Bristol, UK.

Unstackable shapes

To mimic temperature changes in the gel, Royall’s team gradually added in a second polymer, and eventually the colloid flipped into a glass-like state.

This geometry is incapable of slotting together, or tessellating, to form the regular 3D lattice characteristic of a solid. But equally they cannot move around freely because they are larger than the original particles.

Royall thinks that the molecules of real glass takes on the same icosahedral structure, leaving it unable to crystallise into a solid, but not free enough to have liquid-like properties.

‘Metallic glass’

It has been postulated for 50 years that icosahedral structures were behind the properties of glasses, but this is the first time the process has been confirmed in the lab.

“For a long time, no-one has really shown what the structure of glass is,” Royall says, “but we have been able to show how the structure of a glass differs from that of a liquid.”

Creating metals that that form icosahedral structures as they cool could create a new class of “metallic glass” materials, he adds.

Without the ordered crystal structures typical of metals, these materials might be more resistant to metal failure that can happen when layers of crystals shear away from one another. That would be useful in situations where metals are subject to high stresses, such as aeroplane wings.